1. Field of the Invention
The present invention relates to a toner for developing an electrostatic image through a method for forming an image such as electrophotography and electrostatic printing, or a toner for forming toner images through a method for forming an image of a toner jet process, a method for forming an image, and a method for forming a full-color image. More specifically, the present invention relates to a toner employed in a fixing process in which those toner images are fixed under heating and pressure onto a transfer material such as a print sheet, a method for forming an image, and a method for forming a full-color image.
2. Description of the Related Art
As a color image forming apparatus of an electrophotographic process is spread widely, its use has also prevailed in wide variety, leading to a severe demand on image quality. An extremely fine and faithful reproduction of even fine portions has been demanded for a copy of an image such as general photographs, catalogs, and maps. Along with this demand, a demand toward vividness of color has increased, and an extension of color reproduction range is desired. In particular, adoption of such an image forming apparatus has been remarkably expanding in the field of printing, so that a high level of colorfulness, fineness, graininess, etc. equal to or higher than a print quality is demanded for the electrophotographic process as well.
In a recent image forming apparatus of an electrophotographic process employing digital image signals, dots of a constant potential gather on the surface of a latent image bearing member, that is, a photosensitive member to form a latent image, and solid portions, halftone portions, and line portions are expressed by changing a dot density.
However, in this method, a problem tends to occur in that gradation of a toner image corresponding to a ratio of the dot densities of black portions and white portions of the digital latent image cannot be obtained because toner particles are hardly deposited on the dot faithfully and they fall out of the dot. Further, when enhancing resolution by reducing a dot size to improve the image quality, it becomes more difficult to reproduce the latent image formed with minute dots. In addition, an image poor in resolution and in gradation of a highlight portion, in particular, and lacking in sharpness tends to be obtained. Further, irregular dot arrangement is perceived as graininess and becomes a cause of degrading the image quality of the highlight portion.
A method for forming an image, employing a deep color toner (deep toner) for the solid portion and a toner with lower density (pale toner) for the highlight portion, is proposed for alleviating the above problem.
For instance, a method for forming an image employing a plurality of toners with different densities combined has been known (see, for example, JP 11-084764 A and JP 2000-305339 A). Further, JP 2000-347476 A proposes an image forming apparatus combining a pale toner having the maximum reflection density of half or less of the maximum reflection density of a deep toner.
In addition, JP 2000-231279 A proposes an image forming apparatus combining a deep toner having an image density, with a toner amount of 0.5 mg/cm2 on a transfer material, of 1.0 or more and a pale toner having the image density below 1.0. Further, JP 2001-290319 A discloses an image forming apparatus combining toners with a slope ratio of recording densities of a deep toner and a pale toner between 0.2 and 0.5.
Studies of the inventors of the present invention have found out that those conventional techniques may improve the gradation and mitigate the graininess in a low-density region consisting of the pale toner alone, but cannot provide sufficiently satisfactory graininess in a medium-density region where the deep toner and the pale toner are mixed. In addition, those techniques leave some room for contrivances to extend a color reproduction range.
Although some of the publications published up to now included descriptions relating to a method for forming an image using a pale toner, none of the publications described an optimum design of hue and density of a colorant for the pale toner and an effect of the kind and amount of a wax on mitigation of the graininess (roughness) of an image in the low-density region and on expansion of a fixing temperature region.